Abiotic Stresses

Urban trees face numerous abiotic stresses that significantly affect their health, growth, and longevity. These stresses include limited soil volume, soil compaction, air pollution, road salts, heavy metals, drought, mechanical damage, light pollution, and the urban heat island effect, to name a few. The local conditions of urban environments intensify the magnitude of these stressors and their impact on trees. Understanding and managing these cumulative impacts is crucial for the sustainability of urban forests (Collins, 2007).

Soil compaction and inadequate soil volume are persistent challenges for trees in built-up areas. Unfavourable soil conditions and limited rooting space negatively impact sustained tree growth and physiological functions due to reduced oxygen, restrained water and nutrient availability. For example, soil compaction increases bulk density and reduces soil pore space, restricting the growth of fine feeder roots essential for absorbing water and nutrients. In turn, this makes trees more vulnerable to drought and other stressors, which can lead to the premature decline of urban trees over time and threaten the overall health and resilience of the urban forest (Cushing, 2009; Jim, 2023).

Air pollution, including particulate matter, ozone, sulphur dioxide, and nitrogen oxides, impacts urban trees by reducing their photosynthetic efficiency and growth. It has been shown that trees exposed to high levels of air pollution may exhibit symptoms like chlorosis, reduced leaf size, and premature leaf drop, which weaken trees, making them more susceptible to other stresses (Grote, 2016; Moore, 2023).

The use of de-icing salts is another stressor that poses a significant threat to urban trees in Canada. Road salts, primarily sodium chloride, accumulate in the soil, leading to osmotic stress and toxicity. Symptoms of excessive road salt on trees include leaf scorch, reduced growth, and even death, particularly in poorly drained areas (Equiza et al., 2017; Government of Canada, 2015). With regards to road salts, reducing salt application, using alternative de-icing materials such as sand, choosing salt-tolerant species, and designing landscapes to minimize salt runoff are effective strategies that help sustain urban forests (Government of Canada, 2015; Transportation Association of Canada (TAC, 2024). 

Urban trees, particularly those near traffic and industrial sites, often accumulate heavy metals in their tissues, causing toxicity and leading to impaired tree growth. These contaminants can reduce growth rates, cause leaf discoloration, stress trees and increase their vulnerability to pests and diseases. Studies have shown that heavy metals like copper, mercury, manganese, nickel, lead, and zinc are found in higher concentrations in the bark of trees growing closer to streets, contributing to long-term physiological stress and reduced growth (Nechita et al., 2021; Yousaf et al., 2020; Kargar, 2013). It is essential to monitor and manage soil quality regularly, remediate contaminated sites when necessary, and select tree species tolerant of pollutants (Nechita et al., 2021)

Drought is a common stress in urban areas, especially during summer when water availability is limited. Urban trees, already stressed by poor soil conditions and impermeable surfaces, are more vulnerable to drought, leading to reduced growth, dieback, and mortality. The urban heat island effect exacerbates these conditions by increasing temperatures within urban areas and accelerating water loss through evapotranspiration (Dale & Frank, 2022). This phenomenon is particularly concerning in the context of climate change, which intensifies heat waves and further stresses urban trees, weakening them and increasing their susceptibility to diseases and pests (Duinker et al., 2015; Ziter et al., 2019). Selecting drought-tolerant species, implementing efficient irrigation, and using mulching improve the water-holding capacity of urban soils and reduce the impact of drought (Saddle Hills County, n.d.).

Mechanical damage from construction, vehicular impacts, and improper pruning are also common in urban areas. Such injuries become entry points for pathogens, resulting in decay and structural weakness, which can significantly reduce a tree’s lifespan (Krige, 2024). For example, mechanical damage has been identified as a significant threat to the urban forest in Toronto, requiring careful management and mitigation strategies like using physical barriers or fences around trees, pruning trees of concern, and post-construction soil/wound treatment (City of Toronto, 2017; Krige, 2024; Shinwary, 2021; Fraedrich, n.d.). When it comes to reducing mechanical damage, it is essential to implement protective measures, such as tree guards, and to educate the public and professionals about proper tree care practices. Regular inspections and maintenance can also help identify and address mechanical injuries before they lead to more severe issues (City of Toronto, 2017; Krige, 2024; Shinwary, 2021)

Some other overlooked stressors include artificial light and dog urine. Artificial light can disrupt the natural growth cycles of urban trees, interfering with photosynthesis and respiration. It has been shown that excessive light exposure can delay leaf drop, disrupt flowering, and reduce overall vigour, weakening trees and increasing their susceptibility to other stresses (Meng et al., 2022). Using shielded lighting, adjusting light timing, and selecting species less sensitive to light fluctuations can mitigate the effects of light pollution (Meng et al., 2022). Dog urine is an abiotic stressor linked to increasing urban population density and, thus, dog ownership. Studies have shown that though dog urine deposition and “fertilization” are localized due to their high nitrate, ammonium, and phosphorus concentrations, they have a negative impact on soils and trees. Soils impacted by dog urine also have significantly higher salt concentrations (lower osmotic potential), making it harder for trees, especially younger trees, to take up water (De Frenne, 2022). 

Strategies like increasing tree canopy cover, using reflective materials in urban design, and creating green infrastructure that cools the urban environment can be used to mitigate the impact of heat on trees. Green infrastructure solutions, such as rain gardens and permeable pavements, also help reduce heat stress (Dale & Frank, 2022; Ziter et al., 2019). These strategies benefit trees and improve overall urban livability by reducing heat and improving air quality (Dale & Frank, 2022). 

In addition, resilient species that can withstand a range of environmental conditions should be prioritized. Selecting tree species that tolerate urban conditions is crucial. For example, it has been observed that species like Ginkgo (Ginkgo biloba), Honeylocust (Gleditsia triacanthos), Oak (Quercus spp., and Elm (Ulmus spp.), as well as Kentucky Coffee-tree (Gymnocladus dioicus) and Northern Red Oak (Quercus rubra) might show higher resilience against urban stressors like drought, soil compaction, and pollution (Carol-Aristizabal, 2024; Credit Valley Conservation, 2022).

Considering the complex interactions between abiotic stresses and the health of urban trees, integrated management and planning approaches are necessary to maintain resilient urban forests. Regular monitoring and adaptive management ensure the long-term sustainability of urban forests. In addition, landscape designs combined with strategic species selection and tree planting standards can help to minimize salt leakage and soil contamination. 

Abiotic stressors on trees and tree management within such conditions are considered in urban forest management plans and actions in many Canadian cities [see chapter: Urban Forest Management Planning]. Well-planned management that includes regular maintenance activities, such as pruning, watering during dry periods, and monitoring tree health, is essential for managing the cumulative impacts of abiotic stresses. By implementing best management practices in urban forestry, Canadian cities can ensure that their urban forests continue to provide ecological, social, and economic benefits for a longtime.